1. Field Of The Invention
The present invention is generally located in the field of mounting lenses of an LED light-emitting divides module on a substrate, such as a printed circuit board (PCB), such that the lens will be in a defined position vis-à-vis an LED chip of the LED module.
2. Related Technology
The lenses are generally used to achieve a defined radiation pattern and/or smaller viewing angles. The lens can be used in combination with other beam shaping elements such as for example an reflector or an iris.
When lenses are in direct contact with the LED chip (i.e. without air gap) they have to be considered as a primary optics. These type of lenses, where between the light exit from the chip and the lens surface the refractive index does not change, are called immersion type lenses. If there are minor refractive index changes e.g. from silicone with refractive index 1.4 and e.g. Glass with refractive index 1.49-1.65, the lens will still act like a immersion type lens and is basically primary optics. If basically the refractive index on a lens element is the same before entrance and after the exit, the lens is called an non-immersion type lens and can be called secondary optics.
On the other hand, if there is an air gap (or another optical element) and thus a substantial change in the refractice index in the light path from the LED chip to the lens, they are considered as secondary optics.
Lenses as primary optics are common in the field of LED modules and often used as immersion type lens. Such LED modules often consist of a lead frame where the chip is usually placed in a small reflector and the lens is positioned by over molding the LED chip. Typically epoxy resin is used as a material for the lens, which has the disadvantage of limited light stability and yellow-degradation, which is particular an issue for blue and white LEDs. Also glass and silicone resin or silicone rubber as well as polymethyl methacrylate (PMMA) or polycarbonate (PC) or other suitable thermoplastics cana be used, as long as they are transparent or for some application colored and can be brought in the form of a lens.
Such type of lenses do also have the disadvantage that not all of the light is collected due to the fact that light can only be redirected up to an angle of arcsin (1/nepoxy) which is some 40 degrees for a refractive index of the epoxy resin of 1.5. Therefore, for small beam shapes of e.g. 10° only light from +/−45° can be collected due to this limitation (without the use of additional light directing methods which basically also requires bigger lens diameters to become effective.)
For a surface emitting device with Lambertian radiation pattern this means that maximum 50% of the emitted light will be redirected into the desired angular range. Due to total refraction which occurs at the limiting angle, the figure of merit is in reality lower, normally reaching e.g. around 25% depending on the chip size and the lens size.
At higher viewing angles (e.g. lenses with an viewing angle of 90°) a second limit due to molding technology and the demolding process has to be taken into account as for such applications the shapes have to be undercut. Such undercut shapes can not be demolded and are therefore rarely used.
Reflector-based primary optics approaches do also exist. Although with a long parabolic shape of the side walls of the lens and partial metalisation of the side walls very high efficiencies can be achieved, the production of such reflector based LED modules turned out to be difficult.
Refraction and reflection can also be combined to achieve a maximum output of the LED module, wherein the central portion of the light beam is refracted and the side emissions are reflected allowing almost 100% of the light output to exit the LED module. However, the design and production of such combined refraction/reflection LED modules does lead to bigger diameters of the LED package and can therefore only applied when sufficient space is provided in the application.
The third principle of beam shaping in LED modules is using a diaphragm which in terms of figures of merit is inefficient as most of the light emitted from the LED chip will be blinded out.
With thermoplastic materials limits regarding the soldering process do exist and only methods where the lens is protected from the required temperature for the soldering are suitable.
The term ‘secondary optics’ already suggests that another optics is already in place, which primary optic can e.g. be an air gap between the LED chip and the secondary optics. Secondary optics are known which are a combination of an inner refractive optic and an outer reflective (e.g. based on total reflection) optics. The efficiency is basically sufficient as almost all of the light can be directed into the desired angular range except losses of reflection which do always happen on surfaces which change the refractive index. These known parts are usually manufactured by an injection molding process. Therefore, these lenses can not be produced by overmolding, but have to be applied in a second manufacturing step.
FIG. 4 shows a prior art LED module known from JP 2006140281 A. According to this document a LED chip 103 is mounted on a metal stem 102 from which a plurality of leads 101 formed of a conductive material are extracted outside. A lens holder 106 to which a glass lens 107 is temporarily fastened by silicone resin is so welded and integrated on the metal stem 102 as to surround the LED chip 103. Thereafter, silicone resin having translucency and flexibility is injected as the sealing resin 10 into a space formed by the metal stem 102, lens holder 106 and glass lens 107 to resin-seal the LED chip 103 and bonding wires 104.
Also this prior art document JP 2006140281 A relies on a first manufacturing step which is the welding of the LED chip on the metal stem 102 and then a second manufacturing step which is the injection of the silicone resin in order to mount the lens holder 106 and the glass lens 107.